UNIT investigates and combats paranormal and extraterrestrial threats to Earth usually with the help of The Doctor. In the classic series several UNIT personnel (such as the Brigadier) played a major role in the show.

Much is made of the so-called "UNIT Dating Controversy". Fans, novel writers, and even a documentary struggled to try to deal with the controversies, but ultimately, as Ben Aaronovitch said, "There is nothing you can do about [Mawdryn Undead]. It's just stuffed. You just pretend it's taking place in an alternate universe."

As that quote implies, the problem isn't really in the original 1970s UNIT stories (or their predecessor, The Invasion); the controversy was caused by later stories that return to the same characters—most notably the Brig's appearance in Mawdryn Undead and Battlefield, Mike Yates' later reappearances, and one throwaway line from Sarah Jane Smith.

After watching the later stories, it becomes clear that the Brig was definitely dating Doris while she was married to Sarah Jane's uncle, and possibly while he was married to Fiona. Adultery is a controversial subject in any medium, but especially for a 70s kiddie show.

Further, Mike Yates' frequent attempts to date Jo Grant don't fit with the later revelation of his homosexuality. Of course in retrospect, Jo would have made a perfect hag for Mike and the boys to hang out with at The Twist (the local gay bar near UNIT HQ), but it was pretty clear that wasn't what he was after; he was trying to get into her pants. This made a good plotline for, say, Rusty Davies' 2001 show aimed at an adult audience, where a gay man named Davies who falls in love with a lower-class blonde woman named Rose and has to deal with his gay friends not understanding his sudden interest in the boobs, the BBC resisting his desire to revive Doctor Who as a show about Rose, and the challenges of solving mysteries while working as an assistant to a stage magician. But again, for a 70s kids' show, this is quite controversial.

The novels tried to solve these problems, but only made things worse, by inventing such fantastic concepts as "bisexuality" that simply defy the reader's suspension of disbelief.